1. Field of the Invention
This invention relates to a sealing process and sealing apparatus using adhesives to seal electronic packages.
2. Description of the Prior Art
One conventional method for sealing electronic packages uses organic adhesives to attach a package base and a package lid together. An organic adhesive is typically applied to the mating surface of the package or to the lid (or possibly both) and the two parts are put into contact and aligned. Pressure is applied to the package/adhesive/ lid interfaces while the package, lid, and the adhesive therebetween are heated until the adhesive is cured.
The conventional method for sealing electronic packages can use a paste adhesive or a Beta-stageable adhesive for sealing. The paste adhesives typically have a viscosity between 30,000 and 300,000 cps and ideally have flow characteristics which allow the adhesive to flow only as pressure is applied. In this way, an adhesive with paste-like consistency can be applied manually or automatically to the mating surfaces of the package or lid with a syringe, for example, and the adhesive should not spread beyond the mating surfaces to contaminate other surfaces of the electronic package after it is applied. Nevertheless, handling of packages and lids with a fluid or paste adhesive applied thereto is difficult. The paste adhesive will have a tendency to flow even without pressure due to temperature and surface characteristics of the mating surfaces. Moreover, the fluidity of the adhesive is not readily amenable to alignment adjustments made when the package and lid are mated together.
Beta-stageable adhesives are another form of adhesive for sealing packages. Characteristically, beta-stageable adhesives are in a tack-free, semi-solid state and are available in the form of a film or sheet, for example. The semi-solid state is advantageous to sealing packages according to the conventional method because beta-stageable adhesives are tack-free until heated to about 50.degree. C. This is advantageous to the handling of packages and lids until the parts are mated and aligned together, thereby avoiding liquid handling and hazards associated with solvents for clean-up. Also, tooling can be made to cut the adhesive into a preform of any desired dimensional shape for package sealing. The adhesive user can specify the dimensions needed and the adhesive manufacturer will tool up to create the desired shape preforms (typically resembling that of a gasket) usually with a one-time tooling cost per package specification.
Alternatively, the adhesive manufacturer can apply the beta-stageable adhesive to the packages or lids for the user and sell the adhesive according to the cost incurred for preapplying it to the customer's packages or lids. In that case, the customer will typically supply the packages or lids to the adhesive manufacturer for this purpose.
Current state-of-the-art technology relies on the application of mechanical force to the pre-aligned package and lid prior to the application of heat to flow and cure an adhesive. In some instances, lids are initially tacked in place by pre-heating the adhesive to about 80.degree. C. while mating the lid and package. Under such conditions, the adhesive acts as a pressure sensitive adhesive, like cellophane tape, to hold the lid in position while a clamping device is applied. Usually the tacked assembly is allowed to cool to room temperature before it is clamped and placed in an oven to achieve the seal and cure the adhesive.
Regardless of the mode of applying the beta-staged adhesive, essentially all electronic package sealing is accomplished by starting with the assembly of a package and a lid with an adhesive at or near room temperature with mechanical force applied to cause the adhesive to flow and keep the assembly in alignment when heat is applied. The procedure is useful, but typical reject rates due to a condition called "blowout" range from about 3% to as much as 50%. Part of the blowout problem is due to lot-to-lot variations in the adhesives used combined with dimensional variations in the lid or package. However, the major cause of blowout is the thermodynamics of the heat sealing process itself.
When a clamped package and lid having adhesive therebetween is placed in an oven, its temperature will rise to the oven set temperature by conduction, convection, or radiation heating, or a combination of all three conditions. At a temperature of about 50.degree. C. the adhesive will become soft and sticky and the clamping force will cause it to flow and make a gas tight seal. The seal entraps and contains whatever volume of gas is in the package at that moment in time and temperature. However, the assembly continues to warmup to the oven set temperature, which typically is 150.degree. C. to 175.degree. C. This causes the gas pressure in the cavity of the package to rise. At the same time, the higher temperature causes the adhesive to become thinner (lower viscosity) and diminishes its ability to hold the gas in the cavity. Usually, gas bubbles will penetrate the seal during this heat-up cycle, but the fluid adhesive will reflow and "heal" the hole made by the gas bubble.
Typically, beta-staged adhesives used for package sealing will "gel" or set in less than 30 minutes at the normal cure temperatures in the range of 150.degree. C. to 175.degree. C. However, this phenomenon is not instantaneous. The material will thicken and resist flow well before it is cured. If an out-gassing bubble occurs after the adhesive thickens, the hole made by the bubble penetrating through the seal may not heal and the result is a package which will leak because of a blowout. Anomalies in the mating surfaces of the package and lid due to lead penetrations, lack of flatness, or other minor defects usually found in commercial electronic packages and lids further compound the blowout sensitivity. Finally, all beta-staged materials in commercial use in 1990 have room temperature shelf lives of six months or less. This means that the chemical curing reaction is taking place very slowly, but the material is constantly changing. Therefore, at the point of use, each lot of adhesive is different, not only because of normal batch-to-batch tolerances, but also because of the age of the adhesive and the conditions associated with shipping and storage of the adhesive. Superimposed on this are other manufacturing factors like variations in clamping force, oven temperature uniformity, and operator error, so that it is easy to understand why the wide variations in defects and sealing yield losses occur from day to day and lot to lot.
It would be desirable if a process could be developed which is not sensitive to the variations and limitations described above for the state of the art.